It is a long-standing objective of color photographic origination materials to maximize the overall response to light while maintaining the lowest possible granularity. Increased photographic sensitivity to light (commonly referred to as photographic speed) allows for improved images captured under low light conditions or improved details in the shadowed regions of the image. In general, the overall light sensitivity provided by the light sensitive silver halide emulsions in such systems is determined by the size of the emulsions. Larger emulsions capture more light. Upon development, the captured light is ultimately converted into dye deposits which constitute the reproduced image. However, the granularity exhibited by these dye deposits is directly proportional to the grain size of the silver halide emulsion. Thus, larger silver halide emulsion grains have higher sensitivity to light but also lead to higher granularity in the reproduced image. It has been a long-standing problem to provide materials which maximize the response to light of a silver halide emulsion for any given grain size.
For example, it is well known that highly reactive couplers or couplers that form dyes with high extinction coefficients can maximize the response of silver halide emulsions. However, the increased amplification caused by these types of dye forming materials also directly leads to higher granularity.
It is highly desirable to provide imaging materials that would lead to increased photographic speed without having to increase the size of the light-sensitive silver halide grains.
It is well known that couplers that release inhibitors of silver development upon reaction with oxidized developers are useful for controlling photographic characteristics such as granularity, sharpness and color reproduction. These couplers are typically referred to as DIRs (Developer Inhibitor Releasers). In some cases, the inhibitor of silver development is not attached directly to the coupling species, but to a linking or timing group which may delay the formation of the free inhibitor fragment. The development inhibiting fragments released from these couplers are generally either heterocycles with free thiol groups such as mercaptotetrazoles, mercaptotriazoles, mercaptooxadiazoles, mercaptothiadiazoles, etc or nitrogen based heterocycles such as tetrazoles, triazoles, benzimidazoles, benzotriazoles, etc. All these fragments share the common feature that they inhibit silver development in order to produce the desired effects. However, inhibition of development usually leads to loss of photographic speed and at best, results in no change in photographic speed.
Purines used as inhibitor fragments as part of a DIR are disclosed in U.S. Pat. No. 3,933,500, EP 867763A1, EP 867764A1, EP 369486B1 and JP 04-2878942. JP52-154631 describes the use of certain 1,3,3a,7-tetraazaindenes as inhibitor fragments as part of a DIR. EP 220746A2 and DE 3814635A1 discloses the preparation and use of tetraazaindenes as fog inhibitors when released from oxazole type developing agents. JP 60-29390 describes the use of partially ballasted benzotriazoles as inhibitor fragments as part of a DIR. JP 10161287A2 describes the use of a ballasted benzotriazole derivative attached to a transferable dye precursor in a low silver photographic material for use in an amplified development process. U.S. Pat. No. 4,861,701 and FR 2005298 disclose the use of partially ballasted mercaptotetrazoles as inhibitor fragments as part of a DIR. JP 62-245263 describes the use of a partially ballasted mercaptotetrazole as an inhibitor fragment as part of a hydrazide developing agent. None of the above references contain inhibitor fragments that are of sufficient ClogP (as hereinafter defined) to increase the photographic speed.
A common problem with the addition of materials which are strongly adsorbed to silver halide (i.e. inhibitor fragments) directly to a silver halide emulsion is a loss in sensitivity to light. This loss in sensitivity can be magnified if the free inhibitor fragment is present with the silver halide for any period of time at elevated temperatures under typical manufacturing conditions. In order to minimize this undesirable interaction, the silver halide and the free inhibitor fragment are kept separate from each other until mixed just prior to coating. However, this complicates the manufacturing operation, requiring additional steps, equipment and handling and leads to increased cost and waste. It is highly desirable to have materials that can be held together in suspension with silver halide emulsions for extended periods of time, usually at least 5-10 minutes or more, without causing any changes in photographic performance.
A problem to be solved is to provide a color photographic element having improved light sensitivity without significant adverse effect on granularity or ease of manufacture.